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Well he will get hurt alright. But you can help him move on. I have been rejected before twice once by a girl who did it in a mean way infront of everyone... I guess I was inflating her over inflated ego, it took over a year to recover my dignity and gather some courage. 2nd by a girl who did it in a nice way... I am still good friends with her... I totally accepted her decision and moved on in a month.
So my advise is... tell him you are already in a relationship, don't make him think there will be a chance in the future, tell him that you can be friends. Just act normally afterwards so that things don't get awkward. He will accept your decision.
You really can't - which is why you should be as direct and brief as possible without being mean - try to keep his feelings in mind, but be direct and extinguish all hope from the start. You won't want this:
You don't want to be cruel or mean, but you need to be CLEAR, and he's probably going to have some hurt feelings, and you can't really prevent that. Sad to day, a few guys may lash out at you when you reject them - but that's because they're hurt and they're embarrassed and try to show anger instead of pain. You don't have to forgive them, but at least understand where it's coming from and don't take it too personally.